1. Field of the Invention
This invention relates to the recovery of fluid fuels from subsurface deposits of carbonaceous materials, and more particularly to a method and structure for delivering combustion air into the upper end of a rubblized retort for the in-situ retorting of oil shale.
2. Description of the Prior Art
Immense potential sources of carbon-containing compounds suitable as fluid fuels exist in subsurface carbonaceous deposits of oil shale, coal, and heavy, highly viscous petroleum oils. The highly viscous petroleum oil deposits are frequently referred to as tar sands. Because the carbonaceous material in the deposits is either solid as in oil shale and coal or highly viscous as in tar sands, treatment of the carbonaceous deposit to make the carbon-containing compounds fluid is necessary to deliver them from the deposit to the surface. A method of treatment that has been used is to heat the deposit to a temperature at which fluid carbon-containing compounds are formed or the viscosity of heavy oils is drastically reduced. One method of heating the deposit is by in-situ combustion in which a portion of the carboniferous material in the deposit is burned in place by igniting the deposit and injecting air into the deposit to heat oil shale or tar sands to a temperature at which oils of low viscosity are produced or to produce combustible gaseous products from coal.
The very low permeability of oil shale makes it necessary to rubblize the shale to form an in-situ retort through which fluids for heating the shale to a temperature high enough to convert the kerogen to shale oil can be circulated. While sometimes coal and tar sands may be sufficiently permeable for an in-situ combustion process, rubblization of those deposits can be advantageous in reducing channeling through the deposits. One of the methods of forming an in-situ retort is described in U.S. Pat. No. 1,919,636 of Karrick. In the process described in that patent, a vertical central shaft is driven through the oil shale to provide the desired void space necessary for permeability and the oil shale is blasted from the walls of the shaft to fill the shaft with broken oil shale. Other mining procedures for forming a rubblized in-situ retort are described in U.S. Pat. No. 2,481,051 of Uren, U.S. Pat. No. 3,001,776 of Van Poollen and U.S. Pat. No. 3,661,423 of Garrett. Those patents suggest using various mining techniques such as sublevel stoping, sublevel caving, block caving and shrinkage stoping to form an in-situ retort having 5 to 40 percent void space.
Combustion air for the burning of a portion of the shale to liberate heat and thereby form hot combustion gases that are passed through the rubblized shale to produce shale oil is supplied in the process of Karrick through pipes extending upwardly through a retort under construction, across one retort in which combustion is occurring, and into a second retort to discharge air and fuel into the upper ends of the retorts. The pipes are exposed to temperatures resulting from combustion in the retort high enough to seriously weaken the pipes and cause their collapse. Moreover, there will be movement of the rubblized shale in the retort during the construction which can break or collapse pipes. As a practical matter, the very large size of retorts required for economic operations requires combustion air supply passages so large that pipes such as disclosed by Karrick are not feasible.
It is suggested in the Van Poollen patent that combustion air for the retorting of the shale oil be supplied through shafts drilled from the ground surface through the overburden into the upper end of the retort. Some subsidence above the retort resulting from the rubblization of shale in the retort and the subsequent weakening of rock by the high temperatures that occur during the retorting can be expected. The possibility of such subsidence raises a danger of losing the air shaft in the method described in the Van Poollen patent. Another problem with the method described in the Van Poollen patent is that the shale deposits in the Western United States are generally in mountainous regions and the number of acceptable drilling rig sites on the ground surface is limited. Directional drilling of air shafts from the ground surface with sufficient accuracy to open into the in-situ retort at the desired location would be an expensive and time-consuming operation. Subsidence of the overburden would also cause a problem in the method of Van Poollen. Rupture of pipes extending from a central compression unit to the upper end of a shaft for the combustion air injection could easily occur as a result of subsidence of the structure above the retort.